MECHANISMS OF SILICON INCORPORATION IN ALUMINOPHOSPHATE MOLECULAR-SIEVES

The mechanisms of Si incorporation into aluminophosphate molecular sie ves has been investigated using lattice simulation techniques. We cons ider the relative stability of dispersed Si and Si islands, along with changes in the substitution mechanisms in the different structure typ es. Two mechanisms are considered: P substitution by Si (SM2) and the replacement of adjacent Al and P with 2Si (SM3). We demonstrate that a ggregation of silicon will only occur above certain thresholds determi ned by the structure type. Our previous work has demonstrated that sil icon island formation by means of SM3 is energetically unfavourable, a nd thus 2Si and 4Si islands are not expected to form in SAPO structure s. In contrast, the formation of silicon islands by means of a combina tion of SM2 and SM3 has proved to be energetically favourable, leading to 5Si and 8Si islands being stable in the SAPO structure. The instab ility of [Si-O-P] linkages is therefore demonstrated. We also investig ate the stability of Si islands in a number of SAPO structures. We fin d differences in island stability in these different structures, which can be explained in terms of the connectivity of the framework struct ure.